The Universe in the Classroom

An Ancient Universe: How Astronomers Know the Vast Scale of Cosmic Time


With all this in mind, let's now look at what our observations and experiments have revealed about the age of the universe and its contents. We examine each thread of evidence separately, but we will see that they fit together very nicely to reflect the ages we discussed above.

a) The Age of the Expanding Universe

Astronomers can estimate the distance of each galaxy of a certain type from its apparent size or brightness. The smaller and fainter a galaxy appears, compared to similar galaxies, the farther away it must be. We experience the same effect here on Earth - the farther away a car, the closer together and fainter its headlights appear. In addition, there are other ways of measuring the distances to galaxies, using special stars that act like mile-markers.

Astronomers can also determine the speed a galaxy is moving by breaking up its light into its component colors, rainbow-fashion. We call the light spread out like this a "spectrum" and it is something whose properties astronomers are very good at measuring. Christian Doppler showed in 1842 that when a source of light is moving away from us, the motion stretches the waves, slightly changing the colors we see in the spectrum. This Doppler Effect, which applies to all kinds of waves, also explains why a police siren that is approaching us seems to have a higher pitch, and one that is moving away from us seems to have a lower pitch.

When we measure light from distant galaxies we find that their waves are always stretched, indicating that the galaxies are moving away from us. By measuring the stretching, we can determine the galaxies' speeds. Astronomers have been making such measurements since the first decade of the 20th century.

In the 1920's astronomer Edwin Hubble made the remarkable discovery that the speeds at which galaxies are moving away from us are not random, but have a pattern to them. The farther away a galaxy is, the faster it is moving away. This pattern is now called "the expanding universe", since the entire universe of galaxies seems to be moving away.

Astronomers soon realized that they could use this information to measure how long ago the expansion began. To see how we do this, imagine for a minute that you are attending a year-end lunch with your fellow teachers. At the end of the lunch, all the teachers get into their cars and drive away from the lunch in different directions.

Say your home is 120 miles from the lunch site, and you drive home at 60 miles per hour. When you get home at 5 pm, you realize you forgot to look at your watch to see when the lunch broke up. Still, you have all the information you need to figure out when all the teachers started "expanding" away from the lunch. Since you traveled 120 miles at 60 mph, the trip took you 2 hours. Thus you can calculate that you, and all the other teachers, must have left at 3 pm. (To check you might call a number of other teachers and ask them to do the same calculation for their trip home. They may have traveled a different distance, at a different speed, but the time will be the same.)

In the same way, we can find out roughly when the galaxies began their expansion by dividing their distances by their speeds. We find the age of the expanding universe to be between 10 and 15 billion years.


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An Ancient Universe - Table of Contents

Home | Introduction | The Universe: An Overview | The Process of Science | The Ancient Universe - The Age of the Expanding Universe - The Age of the Oldest Stars - The Age of Light From Distant Galaxies - The Age of the Chemical Elements | The Changing Universe - Changes in the Solar System - Changes in Stars - Changes in the Universe | Science and Religion | Resource Guide | Activities

© Copyright 2001, American Astronomical Society. Permission to reproduce in its entirety for any non-profit, educational purpose is hereby granted. For all other uses contact the publisher: Astronomical Society of the Pacific, 390 Ashton Ave., San Francisco, CA 94112.

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